The Zen of Newton

Scroll by Hakui Ekaku

Two hands clap and there is a sound. What is the sound of one hand?

Hakuin Ekaku

Force pairs

When two people push or pull against each other but neither one moves, we rationalized that they were both exerting the same force, just in opposite directions. Can an object exert a force against a person??

Still life

Still life, Paul Cezanne

Still still life?

Still life, Paul Cezanne

Forces are interactions

Every force is an interaction between two objects.

Whenever one body exerts a force on a second body, the second exerts a force on the first.

Furthermore, the two forces are

  • equal in strength
  • but opposite in direction.

What's pushing, pushing back?

Pelican taking off


CorneringCan be thought of getting into a position (leaning and turning), where you're manipulating the earth into pushing you.



Typical air resistance at highway speeds: several hundred pounds.

Two strategies to reduce this....

Drive slower

Driving half the speed cuts the force down by a factor of 4 (one quarter).

Speed limits were dropped 75 mph -> 55 mph in the aftermath of the Arab oil crisis in the 1970s.


  • Arranging the surfaces so they're at a shallow angle to the air stream.
  • Cutting down the overall frontal area.

More important the faster you're going (see "Drive slower").


Airplanes stay in the air by moving forward, and angling their wings to push air down. The reaction force causes them to go up.

The effective "gas mileage per passenger" for airline travel is ~30 mpg for each passenger.

Could airlines save money / fuel by re-designing planes to fly slower, just like automobiles?

Unfortunately, the answer is no: The slower a plane goes, the greater the wing area needed to push down enough air to keep it aloft. This increased air resistance pretty closely compensates for the slower speed, and there's no net savings.

Is space travel possible

without air to push against?Rocket launch


Bring your own stuff* to push against.

*Rocket fuel is both the 'stuff' that the rocket pushes against, as well as the source of energy for doing the pushing.


Small and large objects

BugIf a bug hits my windshield, or rather, if my car windshield hits a bug...Is the force that the car exerts on the bug really equal to the force that the bug exerts on my car?

Physics of Cycling


Forces acting on a cyclist include

  • Gravity,
  • Normal forces, that is reaction forces, from the pavement pushing up on the tires,
  • Reaction forces from the pavement pushing the back wheel forward,
  • Air resistance,
  • Rolling resistance.

Suggested Exercises

Conceptual Exercises in Chapter 4: 10, 11, 21.
Problems in Chapter 4: 17, 18.